In power plants cooled with fresh water the industrial water is discharged for operational reasons via a gravity-fed basin into a receiving body of water. A receiving body of water is to be understood as standing or flowing water into which the industrial water is discharged.
GB 699 491 A discloses a regulating device which is to be used as a bypass in a hydro power plant. The proposed regulation device is designed to enable a constant industrial water level to be set in a gravity-fed basin for varying flow rates of industrial water. To this end a throttle member is connected axially to a float gauge at the industrial water level, so that the throttle member is opened if the industrial water level rises and industrial water will be fed by gravity into a receiving body of water. However the throttle member by itself is too slow for rapid regulation of the industrial water level. Depending on the type of power plant, the flow quantity of industrial water through the gravity-fed basin, depending on the allowed temperature rise of the industrial water, amounts to around 5 to 10 m3 per second. The regulation device is also not able to deal with fluctuations in the industrial water flow rate so that fluctuations in the water level of the receiving body of water can be avoided. Fluctuations in the water level of the receiving body can be problematical for gravity-feed operation of the gravity-fed basin. Fluctuations especially occur as a result of regulated pumps or a number of pumps delivering into a gravity-fed basin.
Strong fluctuations of the industrial water level can be countered by the provision of overflow crests. For this purpose the gravity-feed basin comprises a water basin with a crest over which the industrial water is introduced into the receiving body of water. Only when the water level exceeds the height of the crest is the industrial water discharged into the receiving body of water. Depending on fluctuations in the water level of the receiving body, the water flowing over the crest falls many meters downwards in order to mix with the water of the receiving body. The crest can be formed as a linear crest or for example by the upper edge of a gravity cylinder rising from the floor of the basin, as is described for example in DE 31 03 306.
The gravity cylinder described in DE 31 03 306 serves to discharge waste water, e.g. from a power plant, through a gravity-fed basin into surface water. In this invention numerous vertically-aligned guide plates are provided in the gravity cylinder, through which the amount of air introduced into the industrial water is to be increased, so that the industrial water to be discharged will be enriched with oxygen.
Even if no measures are provided for feeding air into the industrial water, a large amount of air is introduced when the water falls vertically which, depending on the water quality, leads to foam formation to a greater or lesser degree. This introduction of air or formation of foam is generally unwelcome both for authorities and for local residents.
Under certain circumstances there is also the danger of stable foam forming which can also lead to significant operational difficulties.
Measures which are designed to reduce the introduction or air or the formation or foam are therefore generally tried out and tested during the commissioning of a gravity-fed basin. Such measures might include temporary coverings, the injection of chemicals or venting or pipe systems. The latter in particular can result in enormous additional costs. There can also be recourse in such systems to fixtures based on different principles.
To reduce foam formation EP 1 693 094 proposes specially-embodied cylindrical gravity cylinders. Foam formation is to be avoided in such cylinders by the exit opening of the gravity cylinder being arranged below the liquid level of the receiving body of water and also by plates being arranged horizontally in the gravity cylinder to provide flow resistance. Such fixtures can only be optimized in these cases to specific ranges of water level for example. With fluctuating amounts of industrial water permanent fixtures present a risk of malfunctions. In addition, especially the gravity-fed basins with various fixtures to some extent need a comparatively large area.
When a gravity-fed cylinder is used there is also the danger of a vortex which is caused by a swirling of the water flowing into the gravity-fed cylinder. A swirl generally leads to eddies in flowing water, which in its turn leads to increased aeration.